1. Field of the Invention
The invention relates to the field of liquid crystals and more particularly to the field of electro-optical display means utilizing electrically excited liquid crystals to provide a changeable visible display.
2. Prior Art
Many liquid crystals for use in electrically excited displays are known in the prior art. When excited by low frequency a.c. signals these crystals display dynamic scattering. In dynamic scattering incident light is scattered by internal scattering centers within the crystal.
As the excitation frequency is increased, the voltage required to maintain dynamic scattering increases. However, a fixed excitation voltage yields a fairly sharp frequency cutoff and for excitation at frequencies above the cutoff frequency no dynamic scattering takes place.
It has been observed that the cutoff frequency is a function of the temperature of the liquid crystal and is approximately halved for each 10.degree.C drop in temperature. This effect restricts either the temperature range over which a given crystal can be used or the excitation frequency which can be used. Although this is not an electrical problem in systems where very low frequencies are available, it presents major problems in microelectronic circuits where low frequency oscillators are unduly bulky and expensive to fabricate. In addition, when the excitation frequency is in the neighborhood of 25 Hertz, a liquid crystal display flickers.